I think you've misunderstood what PG was saying. In this analogy, Y Combinator and Basecamp are the products for sale and HN is a promotional channel. PG's point is that Y Combinator benefits from Hacker News in ways other than the direct "I clicked on the link from Hacker News and then immediately took an action that made money for Y Combinator."

That's not important to the argument. If we made $300/month directly from HN, it still wouldn't be worth it financially.